Receptor mediated activation of many cell types is followed by motility related events. In T lymphocytes, lateral redistribution of surface receptors is accompanied by aggregation of actin and myosin in cytoplasmic subcaps and both are impaired in T cells from older individuals. Age- related changes both in basal levels of filamentous actin and in further polymerization of actin after activation of resting T cells from C57BL/6 mice with Concanavalin A were previously documented. Because of technical problems using flow cytometry to document receptor mediated actin polymerization in populations of CD4 plus CD8 positive cells, a new procedure was developed to isolate CD4 and CD8 T cells immunomagnetically. Resting lymphocytes, isolated with discontinuous Percoll gradients, were incubated with magnetic beads coated with specific monoclonal antibodies to mouse T lymphocyte surface antigens. Coated beads were less efficient than T-cell recovery columns in removing B lymphocytes. Resting T cells were then incubated with CD4 or CD8 antibodies or antibody-coated beads. CD4 or CD8 positive cells, negatively selected in a magnetic field, were 98-lOO percent viable, and contamination by the eliminated cell eliminated types was less than 1.0 percent. The cells were functional and responded to stimulation with the polymerization of actin to levels observed previously in unseparated resting T cell populations.